It is well known to use the speed of sound in a gas for measuring its temperature/humidity or its composition since the speed of sound is only influenced by its temperature and its composition. Measuring the speed of sound can therefore yield the temperature for a gas with known composition, or the composition of a gas at the known temperature. The measurement can be performed with ultrasonic sound or none-ultrasonic sound. Thus, the term “sound” includes both kinds of sound. If the temperature of the gas is known, it is also possible to determine the humidity. The speed of sound is usually measured by determining the propagation time, i.e. the time an acoustic signal to travel the distance between a sender and a receiver. This can be done by sending pulsed signals and measuring the delay for the signal to be detected at the receiver, or by sending a continuous signal and measuring the phase angle between the excitation of the sender and the signal of the receiver. The use of speed of sound measurements is especially appropriate in cases where other measurement principles cannot be used, for example for measurements of the humidity over a large temperature range, for example 30 to 300° C. and/or at specific humidity between 0 and 100%.
The sensitivity of speed of sound measurements based on the measurement of the phase angle between sender signal and receiver signal is a function of L·f, with L the distance between sender and receiver, and f the operating frequency of the sender. In some cases the measurement range is such that the measured phase angle span exceeds 2·π, so that the measurement result becomes ambiguous. In cases where a speed of sound measurement has be done across the diameter of a tube with very high temporal resolution (for example, to measure the temperature variations of a highly dynamic process), f is given by the required temporal resolution and cannot be decreased. L is given by either the diameter of the tube, or by the near field limit of the ultrasonic sender and cannot be decreased at will.
It is therefore an object of the invention to propose a method and an arrangement which is able to measure gas characteristics, especially the gas composition, the temperature and/or the humidity, which overcomes the above drawbacks and allows a measurement with high accuracy of a gas having large temperature variations.
This object is solved according to the method for measuring the speed of sound and speed of sound-based gas sensor arrangement described herein.